Plug one end of an instrument cable into the output of a guitar. Then plug the other end of the instrument cable into the input of a guitar amplifier. If you are using a guitar amplifier that is not connected to a speaker cabinet, connect the speaker cabinet to the amplifier by plugging a speaker cable into the output of the amplifier and into the input of the speaker cabinet.
Plug a Bass guitar into a bass amp the same way a guitar is connected to a guitar amp. If the bassist does not have a bass amplifier, the bass guitar can be plugged into a PA system. For best results when plugging a bass guitar into a PA system, a direct box should be used. When a bass guitar is plugged into a direct box, the direct box converts the high impedance signal from the bass guitar and converts it into a low impedance signal compatible with the inputs of the PA. To connect a bass guitar to a PA system through the use of a direct box, plug one end of an instrument cable into the output of your bass guitar and plug the other end of the instrument cable into the input of the direct box. Then plug one end of an XLR cable into the output of the direct box and plug the other end of the XLR cable into an input of your PA system. After doing so, adjust the input gain of the PA channel you plugged the bass into to adjust the volume level of the bass.
Plug a Keyboard into the PA system the same way a bass guitar is connected to a PA system. However, if your keyboard has two outputs (stereo), then you'll need two instrument cables, two direct boxes and two XLR cables. Pan the PA channels that the keyboard is connected to hard left and hard right when connecting a keyboard with a stereo output.
Set up microphones for the vocalists and any other instruments you need to amplify. Do so by plugging an XLR cable into the output of a microphone and into the input of a channel on the PA system. Then adjust the input gain of the PA channel to set the volume level of the microphone within the mix.